1. Field of the Invention
The present invention relates generally to a turret type machine tool having an automatic tool changing apparatus and more particularly to an improved machine tool which may successively perform special and general purpose machining operations by selectively using a multiple spindle unit and a single tool spindle unit associated with the automatic tool changing apparatus such that both high efficiency and versatility of the machining operations may be obtained.
2. Description of the Prior Art:
In a production system wherein fifty to five hundred workpieces are machined as one lot, a machine tool, known as a machining center, which is equipped with a tool spindle unit and an automatic tool changing apparatus associated therewith has been generally used to automatically machine many portions of each of the workpieces by using a conventional cutting tool, such as a drill, reamer, milling cutter, boring tool, or the like. However, in spite of the adaptability of the machine to a variety of machining operations, the machine tool is relatively restrained in increasing the machining efficiency because one tool is always effective to machine only one of many machining portions of the workpiece at a given time.
For the foregoing reason, recently there have been developed a few kinds of gang head type machining centers which, in addition to the automatic tool changing apparatus for the single tool, are also equipped with automatic gang head changing apparatus. However, in such a machine tool, the storage apparatus for storing a plurality of gang heads or multiple spindle units is completely independent of an operating position where a spindle head adapted to support selectively a gang head and a single tool performs the machining operations, and is positioned far apart therefrom. Therefore, a large capacity for storing many gang heads can be provided, but the head changing apparatus which transfers a gang head selected out of the storage apparatus into the operating position and also restores another head removed therefrom into the storage apparatus becomes complex in construction and further requires a lot of time in the head changing operation. As a result, such a machine tool system has become huge in size and is not as of yet satisfactory as to high efficiency. That is to say, the machine tool system of the prior art is available for mass-production, but is not completely suitable for a subjective production system.